This is a revised application requesting five years of support to continue work on two projects focussing on the synaptic organization and functional properties of brain descending systems controlling limb movements in primates. Both projects utilize the technique of spike-triggered averaging of EMG activity to identify neurons that produce postspike facilitation (PSpF) of EMG activity. PSpF is evidence that the triggering neuron is synaptically linked to motoneurons. We refer to cortical neurons that produce PSpF as corticomotoneuronal (CM) cells and red nucleus neurons that produce PSpF as rubromotoneuronal (RM) cells. Spike-triggered averaging of EMG activity is a powerful tool for identifying the target muscles of premotor neurons and quantifying the strength and latency of output effects on those muscles. Most importantly, it can be applied in awake monkeys where relations between firing rate and various aspects of active and passive movement can be investigated. The objective of the first project is to test the hypothesis that following damage to the corticospinal system, rubrospinal neurons and spared corticospinal neurons undergo changes in their properties that contribute to the recovery of motor function. To test this hypothesis we will compare the postspike effects and discharge properties of RM and spared CM cells in pyramidal tract lesioned monkeys with pre-lesion measurements of RM and CM properties obtained from the same monkeys. Cross-correlograms between the discharges of RM or CM cells and single motor units spike trains will also be used to quantify the strength of synaptic linkages. The second project focuses on two specific aims. One is to document the strength of rubrospinal influence on proximal as well as distal muscles of the forelimb. Work in the past has focused on relations to distal muscles. Second is to investigate a fundamental question concerning the output organization of descending systems. Do some cells of descending systems facilitate functional synergies of muscles that are not direct anatomical synergists but, nevertheless, coactivate during specific motor tasks? Experiments in this proposal will test the extent to which RM cells facilitate combinations of muscles acting in functional synergy at different forelimb joints. These experiments have general implications for the mechanisms of neural plasticity following brain damage and for the organization of descending systems underlying volitional movement.